FIG. 2 shows a production process of a prior art distributed feedback type semiconductor laser device with the use of a conventional diffraction grating production method. In FIG. 1, the reference numeral 1 designates a substrate comprising n type InP, the numeral 11 designates a light guide layer comprising InGaAsP, the numeral 12 designates an active layer comprising InGaAsP having a longer wavelength than that of the light guide layer 1. The numeral 6 designates a cladding layer comprising p type InP. The numeral 7 designates a contact layer comprising p type InGaAsP. The numeral 8 designates an insulating film. The numeral 9 designates a P electrode and the 10 designates an N electrode.
The production process will be described.
At first, an InP substrate 1 is etched with a mask which is obtained by patterning a photo resist by an interference exposure method thereby to obtain a diffraction grating as shown in FIG. 2(a). Thereafter, a crystal growth is conducted on the substrate 1 on which a diffraction grating is produced by such as a general liquid phase epitaxy thereby to produce a light guide layer 11, an active layer 12, a cladding layer 6 and a contact layer 7 as shown in FIG. 2(b). Thereafter, an insulating film 8 is deposited on the contact layer 7, and a portion thereof is removed in a stripe configuration and a P electrode 9 and an N electrode 10 are produced to complete a distributed feedback type semiconductor laser shown in FIG. 2(c).
The device will be operated as follows.
In the semiconductor laser produced as above, a current is flown from the P type electrode 9 to the N type electrode 10. When the current exceeds a threshold value, a laser oscillation occurs due to the induced emission in the active layer and the feedback of the light of a wavelength determined by the diffraction grating.
In the distributed feedback type semiconductor laser of such a construction, according to "Coupled Wave Theory of Distributed Feedback Lasers" (J. Appl. Phy. Vol. 43, No. 5, May 1972, p. 2327), as a light coupling an Index Coupling in accordance with the periodicity of the refractive index occurs, and there arise two modes which give the lowest threshold gain, and a single wavelength oscillation is not likely to arise. Therefore, in order to make the laser oscillate at a single wavelength, a portion at which such as a .lambda./4 shift occurs in the phase of the diffraction grating is required to be provided in the way of the diffraction grating, and this makes the production process difficult and lengthened. Furthermore, in the prior art method of producing a distributed feedback type semiconductor laser, the surface of the diffraction grating is made an exposed surface because the crystal growth is conducted after the production of the diffraction grating. The active layer is produced close to this surface, and a light absorption occurs caused by such as crystal defects at the exposed surface, thereby lowering the light output efficiency and quickening the deterioration, which leads to a short life.